While most molecules have paired electrons in consequence of covalent bonding, some molecules—including free radicals—have electrons that are not paired. Paired electrons have opposite spins (Ms=+/−½) that cancel out net magnetic moments. Unpaired electrons have spins that can interact with magnetic fields.
Unpaired electrons in molecules will resonate in a magnetic field. Electron Paramagnetic Resonance Spectroscopy (EPR), sometimes known as Electron Spin Resonance Spectroscopy, takes advantage of this effect to quantify and determine environments of the unpaired electrons. This is done by applying a magnetic field to a substance, which may be located within a human or animal subject, to align spins of any unpaired electrons in the substance. Once spins are aligned, a response of the spins of the unpaired electrons in the substance to radio-frequency electromagnetic radiation at and near a resonant frequency is measured. The resonant frequency and amount of absorption of the electromagnetic radiation is often dependent on the local environment of the unpaired electrons in the molecule as well as the applied magnetic field. The resonance results in such effects as a spike in a radio-frequency absorption spectrum of the substance in a magnetic field.
An EPR spectrum is often acquired by placing a sample in a magnetic field, holding a frequency of a radio frequency source and absorption measuring device constant, and making repeated measurements of response of the sample to the radio frequency energy while sweeping the intensity of the magnetic field. It may also be obtained by repeated measurements made while holding the magnetic field intensity constant and sweeping the frequency of the radio frequency source and measuring device.
Unpaired electrons are naturally found in small quantities in chemicals, such as free radicals, that are found in biological materials. For example, free radicals are produced during, and have importance in oxidative energy production by mitochondria.
It is known that the hydroxyapatite in tooth enamel and keratin in structures, such as fingernails, develop and retain unpaired electrons capable of producing an EPR signal when teeth and fingernails are subjected to ionizing radiation.
In the case of teeth, this EPR signal is roughly proportional to the mass of tooth enamel and to the total radiation dose received in that mass since the tooth formed. This radiation-induced signal has a long half-life.
In the case of fingernails, there is also a radiation induced EPR signal (RIS) having a moderate half-life of at least a few weeks. The RIS is of an intensity that is a function of radiation dose over a range that extends from radiation doses likely to be survived by a subject without treatment, through radiation doses that require medical treatment of a subject for survival, to radiation doses that are fatal to the vast majority of subjects. Ionizing radiation that can create an RIS includes x-ray and gamma-ray radiation such as that emitted by an operating nuclear reactor or a nuclear weapon detonation, as well as radiation emitted by fission products produced by nuclear weapons and reactors.
If the fingernails are clipped, there is also a mechanically-induced EPR signal (MIS) caused by molecular bonds broken when the fingernail is subjected the mechanical stresses of clipping. The MIS is believed to be due at least in part to shearing of Sulfur-Sulfur bonds between cysteine residues of the keratin in the fingernail. Breaking of these bonds leaves a radical that becomes stabilized. This signal is caused in part by the bending fingernail clippings undergo while their edges are being cut, as well as the shearing of cut keratin at the edges of the clipping.
While some decay is seen in the MIS signal, the MIS has a residual component that occurs at similar frequency and magnetic field strength as, and shows some similar characteristics in spectral shape to, any RIS that may be present.
While nuclear reactor operators typically carry dosimeters for measuring radiation that they may be exposed to in a work environment, members of the public, emergency services organizations, and armed forces rarely carry such dosimeters. In the event of nuclear accident, terrorism, or warfare, it would be desirable to measure recent radiation exposure of people exposed to such events. It is proposed that measurement of EPR of fingernails and teeth may provide dosimetry in such people.
Past attempts of dosimetry using EPR of fingernail clippings have found confusion between the RISs and the MIS to be an issue. Not only are components of the MIS found at the same frequency-magnetic field combination as the RIS, but the MIS is of intensity sufficient to obscure the RIS for much of the dose range of interest for triage for acute effects of ionizing radiation. If EPR of fingernail clippings is to be a practical method of dosimetry, it is desirable to find improved ways of reducing interference from the MIS, or of better extracting the RIS component from an overall EPR signal.
Past efforts to reduce the MIS have included soaking the fingernail clippings in water or sodium thioglycolate solutions, these treatments have been found to significantly reduce MIS by allowing radicals at the edges of cut nails to react. The literature and our experiments suggest, however, that the RIS may also affected by soaking and our experiments with water-soaking have not given reproducible and accurate dosimetry results.
Past techniques for measuring EPR signal in teeth have required extraction of a tooth, a procedure unpopular with victims of nuclear disasters.
In an international climate where perpetual enemies, India and Pakistan, are both nuclear powers, where North Korea has nuclear weapons and Iran—a country that has threatened Israel, a country widely believed to be a nuclear power—may soon acquire them, the risk of a nuclear attack or terrorism is increasingly significant. Further, with recent high oil prices and a worldwide resurgence of nuclear power to produce electricity without emitting greenhouse gasses, there is a significant risk of nuclear accident. Nuclear accidents have also resulted from improper disposal of radioactive materials, such as radiation treatment machines.
In nuclear attack, nuclear improper-disposal events, and nuclear accident, there may be people potentially exposed to ionizing radiation while not carrying previously-issued dosimeters. Both nuclear attack and nuclear accident could be mass-casualty situations with several hundred to tens of thousands of people potentially exposed to radiation.
The Chernobyl, Goiânia, Hiroshima, and Nagasaki events each involved at least some deaths from acute radiation syndrome, as have other events. These events also generated demands for measurements of radiation exposure in many thousands of individuals of widely varying radiation exposure, resulting in a substantial stress on the medical systems.
In mass casualty situations there is often mass hysteria, where large numbers of ‘worried-well’ people physically unaffected by the event may believe that their lives are in danger and may even exhibit psychosomatic symptoms of exposure. This phenomenon is expected to occur in nuclear events such as nuclear attack, terrorism, or accident. Further, there are likely to be limited medical facilities available after some such events—treatment of everyone, the ‘worried well’ as well as the exposed, is not expected to be possible immediately after a major event.
It is desirable to be able to rapidly sort large numbers of people into categories which may include: those who are ‘worried well;’ those with minimal exposure-possibly sufficient to cause increased cancer rates or otherwise need follow up—but who will not need treatment for acute radiation sickness; those who have received significant exposure but should recover from acute radiation sickness with conventional therapy; those who should recover from acute radiation sickness with aggressive therapy such as marrow transplants; and those who will probably die regardless of treatment. In the short term, treatment can then be focused upon those groups who most likely will benefit from the treatment. The process of sorting people according to radiation dose or other injuries into treatable, urgently treatable, or untreatable groups is known as triage, and was formalized for non-radiological injuries by the French army as a way to handle the large number of casualties generated on World War I battlefields. Further, if radiation dose can be approximately quantified, this information can be used to help guide patient transport and treatment by determining which people will likely survive with simple supportive care, which will need advanced care such as transfusions, and which will need more drastic measures such as bone-marrow transplant to survive.
In the event of nuclear attack, communications are likely to be disrupted over a large area. In particular, centralized databases, remote locations, the internet, and the cell and land-line telephone networks are likely to be nonfunctional or unreachable.
Existing technologies for determining which people of a population have been exposed to large doses of radiation include a differential blood count; neutrophils decrease in number because of bone marrow suppression and lack of replacement, while lymphocytes may undergo apoptosis. Unfortunately, not only do such counts require repeated measurements made by skilled medical staff, but baseline counts are unlikely to be available for the majority of people needing screening and both neutrophil and lymphocyte counts may undergo drastic changes from other causes ranging from HIV infection and stress to infection. A better method of triaging the potentially radiation exposed is needed.